The invention relates to a duplexer in which, in the transmission path, a transmission signal is led from a transmission amplifier to the antenna and, in the reception path, a reception signal is led from the antenna to a reception amplifier. In order to be able to transmit and receive simultaneously, different frequency ranges are used for the transmission signal and the reception signal. In order that the transmission signal does not interfere with the very much weaker reception signal, a reception filter is provided in the reception path, which reception filter passes the reception signal and greatly suppresses the transmission signal. An isolation of 50 dB to 60 dB between the reception path and the transmission path is typically required in the transmission frequency band.
In the case of closely adjacent transmission and reception frequency ranges, the isolation in the transmission frequency range is determined by the selection of the reception filter, which is usually embodied as a bandpass filter. If the reception frequency range lies above the transmission frequency range, the selection is predetermined by the gradient of the lower reception filter edges. With a predetermined reception filter bandwidth and the required impedance matching, however, the selection cannot be increased arbitrarily owing to the dictates of design. With the aid of inductances, an improvement in the isolation of approximately 10 dB can be achieved by means of a pole shift. However, this method can be used only to a limited extent in the case of duplexers having a small separation between the transmission and reception frequency bands of only approximately 20 MHz and at transmission and reception frequencies of approximately 1.9 GHz, since the lower reception filter edge is flattened. Furthermore, the inductance values have fluctuations that reduce the transmission range isolation specified as typical.